In measuring the particulate content of a sample gas stream such as, for example, the exhaust of a diesel engine, it has long been the practice to direct the sample stream through a filter assembly containing a removable filter element having suitable particle retention characteristics. Such filter assemblies have included two-section filter housings which have been fastened together by a threaded sleeve that is loosely attached to one housing section and that screws onto mating threads on the other housing section. Such filter assemblies have several shortcomings which make their use inconvenient and which can give rise to inaccurate measurements.
One shortcoming is that the threaded fastener takes a relatively long time to disassemble and reassemble during the course of replacing a filter element. Another shortcoming of the above type of filter assembly is that the sustained vibration that is incident to the unscrewing of the threaded fastener can dislodge particulate matter from the filter element and thereby introduce errors into weight measurements made thereon. Still another disadvantage of the above type of filter assembly is the tendency of the threads to become stripped or clogged with particulate matter during the course of repeated openings and closings. Finally, threaded fastener type filter assemblies cannot be conveniently connected in series; multiple stage filter structures can be provided only by providing a multiplicity of complete filter assemblies, each independently connected to the tubing through which the sample is supplied.